1. Technical Field
This invention relates to a carbon black graft polymer, a method for the production thereof, and a use thereof. More particularly, this invention relates to a carbon black graft polymer which exhibits excellent stability of dispersion to various media. This invention also relates to a carbon black graft polymer which, when added to a varying medium, can manifest fully satisfactorily such qualities as coloring property, imperviousness to light, sensitivity to light, developability with an alkali, and insulation. This invention further relates to a carbon black graft polymer which can be used advantageously in a black light-curing resin composition such as, for example, the black matrix-forming material in the color filter.
This invention further relates to a method for the treatment for surface modification of minute solid particles such as pigments represented by carbon black, magnetic powder, and ceramic powder to improve the minute solid particles in dispersibility in various media such as water, organic solvents, or organic macromolecular compounds, and particularly to a method for the treatment for surface modification which allows a wide range for the selection of the polymer component to be used in the surface modification and implements the surface modification thoroughly by the use of the polymer component in a relatively small amount.
2. Prior Art
The minute solid particles such as pigments represented by carbon black, magnetic powder, and ceramic powder are used more often as incorporated in various compositions in various applications for the purpose of imparting thereto such functions as coloration, reinforcement, and filling than they are used alone in their unmodified form.
Most of these minute solid particles, particularly such minute solid particles as have a primary particle diameter of the order short of submicrons, exhibit weak affinity for other substances such as, for example, water, organic solvents, and organic macromolecular compounds as compared with the cohesive force generated among their individual particles and, therefore, tend to succumb to secondary cohesion. When the minute solid particles are expected to acquire excellent properties in such various compositions as mentioned above, therefore, how uniformly they are dispersed in a given composition poses itself a problem. For the solution of this problem, numerous methods for attaining uniform incorporation or dispersion of minute solid particles in a solid or liquid medium by coating the surface of minute solid particles with a varying surfactant or resin thereby heightening the affinity thereof for the medium have been under study.
As concerns carbon black, for example, (1) the carbon black graft polymer which is obtained by polymerizing a polymerizable monomer in the presence of carbon black has been attracting attention because the hydrophilicity and/or oleophilicity thereof can be suitably varied by appropriately selecting the kind of the polymerizable monomer (JP-B-42-22,047, JP-B-44-3,826, JP-B-45-17,248, and U.S. Pat. No. 3,557,040).
Then, (2) the carbon black graft polymer which is obtained by the reaction of a polymer containing a reactive group such as epoxy group or aziridine group in the molecular unit thereof with carbon black has been proposed (JP-B-02-24,868 and JP-B-06-27,269).
The method (1) mentioned above, however, produces the carbon black graft polymer with a low yield ranging from several % to ten-odds %. The greater part of the polymerization product exists in the form of a vinyl type homopolymer. The efficiency with which the carbon black graft polymer provides a required surface treatment for carbon black, therefore, is extremely low. The product of this method, therefore, does not manifest such high affinity for other substances as is expected and frequently assumes such a dispersed state as is variable with the condition of incorporation or dispersion.
The carbon black graft polymer (2) mentioned above is found to manifest fairly improved dispersibility in various substances. Though the polymer used for this carbon black graft polymer has a functional group reactive with carbon black, the polymer chain thereof solely has either oleophilicity or hydrophilicity. This carbon black graft polymer, therefore, encounters difficulty in fully satisfying frequently contradictory requirements, i.e. the efficiency of grafting with carbon black and the improvement of the dispersibility in various media which the grafted polymer chain is intended to impart.
It has, therefore, entailed such problems as 1 allowing no ample addition to the carbon content in the carbon black graft polymer, 2 imparting to the carbon black graft polymer no fully satisfactory dispersibility in media of high polarity such as, for example, alcohols and cellosolve type solvents or in media of low polarity such as, for example, hydrocarbon type solvents and silicone type solvents, and 3 permitting no easy grafting in an organic solvent.
JP-B-06-27,269 mentioned above also discloses a method for producing carbon black graft polymer by stirring to mix carbon black and a polymer containing in the molecular unit thereof 1 to 2 epoxy groups and/or thioepoxy groups per molecule at a temperature in the range of 50-250.degree. C. thereby inducing them to react with each other. This method is found to permit production of a carbon black graft polymer which excels in dispersibility in various substances because the reactive group present in the molecule of the polymer reacts with the surface functional group of carbon with very high efficiency. The product with improved quality which is obtained by melting and kneading minute solid particles with a reactive polymer and consequently grafting a polymer chain to the surface of the minute solid particles as mentioned above promises to acquire very high dispersibility.
When the minute solid particles and the reactive polymer are melted and kneaded by means of a kneader or a ball mill, for example, it becomes difficult to coat the entire surface of the minute solid particles uniformly with the polymer graft chain when the mixing ratio of the reactive polymer to the minute solid particles is lowered. As an inevitable consequence, therefore, it occurs not infrequently that the content of minute solid particles in the produced surface-modified minute particles is small and the essential characteristics inherent in the minute solid particles are diminished. If the reactive polymer has a low molecular weight or has a large number of reactive groups per molecule, for example, the grafting will either proceed insufficiently or entrain formation of gel. As aptly evinced by this fact, the reactive polymer which is usable has many restrictions such as, for example, the number of functional groups and the molecular weight of the polymer. In order for this method to produce a dispersion of expected quality, it necessitates an extra procedure of causing the mixture resulting from the melting and kneading treatment to be dispersed in a dispersion medium. The method, therefore, has the problem of complicating the process thereof.
It has been also known to the art to adopt a procedure of adding a suitable dispersion stabilizer to the dispersion medium and physically stirring the resultant mixture for effecting necessary dispersion for the purpose of producing a dispersion of such minute solid particles as induce secondary cohesion mentioned above. For the sake of uniformity of the dispersion, it is necessary to consider microscopic mixture based on minute turbulent motions as well as macroscopic mixture. Heretofore, as means to obtain the uniform dispersion, various wet dispersing devices and wet pulverizing devices have been proposed which give appropriate contrivances to the shape of a vessel to hold the fluid to be treated, the shape, number, relative position, and revolution number of stirring blades, the presence or absence of the installation of baffle plates or partitions, and the use, shape, and material of dispersing media such as balls or beads to be incorporated in the interior of the vessel.
Of these devices, those of the type which generally use both stirring blades and the dispersing media have heretofore found utility in numerous applications on account of their capability of forming a state of relatively satisfactory dispersion. The dispersing devices of this type have been disclosed in JP-B-59-22,577, JP-B-02-27,018, JP-A-02-68,151, JP-A-03-101,820, JP-B-07-12,441, JP-B-07-4,552, JP-A-04-32,634, JP-B-06-73,620, JP-A-05-38,425, JP-A-04-281,855, JP-A-06-351, JP-A-06-212,896, JP-A-06-210,148, and JP-A-06-134,271, for example.
The preparation of the dispersion of minute solid particles by the use of such a wet dispersion device or wet pulverization device as mentioned above, however, is implemented by stirring and mixing the minute solid particles with a liquid dispersion medium at room temperature in the device mentioned above and is consequently enabled to form a state of dispersion by dint of the external physical force arising from the stirring and mixing. This preparation, therefore, has the problem of imparting to the produced dispersion such stability as hardly deserves to be called sufficient and necessitating a long time for the treatment of dispersion.
Incidentally, the color filter which is used for a color liquid crystal display device, for example, is provided in at least two kinds of color layers with fine colored areas as picture elements. Heretofore, it has been customary to form a black matrix for the purpose of intercepting light between these picture elements and conferring improved contrast on a displayed image.
The black matrix of this nature is generally composed of minute patterns of a thin film of a metal such as Cr, Ni, or Al on a glass substrate, and fabricated by depositing the thin metal film on the substrate by the vacuum evaporation method, sputtering method, or vacuum film-forming method, for example, and then patterning the thin metal film by means of the photolithographic technique.
Since the black matrix thus produced is expensive on account of the complicacy of process of its fabrication, however, it inevitably entails the problem that the color filter itself which uses this black matrix is likewise expensive.
When the color filter provided with the black matrix using a thin film of such metal as Cr is incorporated in a transmission type display, since the surface of the thin metal film has high reflectance, the transmission type display incurs the problem that when an intense external light impinges on the filter, the reflected light is so intense as to impair the quality of the display conspicuously.
For the purpose of solving the problem encountered by the black matrix using such a thin film of metal as mentioned above, various methods have been proposed for forming a black matrix with a resinous composition containing a coloring agent.
JP-A-02-239204, for example, proposes a method for forming a black matrix with a material prepared by dispersing such a light blocking material as carbon black in a polyimide type resin. This method is capable of producing a black matrix of high reliability because the reflection of external light occurs sparingly and the polyimide type resin as the basis has high resistance to heat. This method covers a procedure which comprises coating the substrate with the polyimide type resinous composition containing the light blocking material, drying the applied layer of the composition, then further coating the dried layer with a photoresist, patterning the applied layer of the photoresist, etching the underlying layers through the patterned photoresist, and peeling the resist. Thus, this method is still deficient in terms of simplification of process and reduction in cost.
An attempt has been under way to develop a method for forming a black matrix of a stated pattern by incorporating a black pigment such as carbon black in a photo-curable resinous composition, applying the resultant mixed composition to a substrate, drying the applied layer of the composition, fixing a mask of a stated pattern to the dried layer, exposing the deposited layer to light through the pattern in the mask, and developing the exposed layer.
When the amount of the carbon black to be incorporated is increased for the purpose of imparting to the black matrix thorough imperviousness to light, it becomes extremely difficult for the mixed composition to be cured by light because the light transmittance in the ultraviolet zone in the neighborhood of 400 nm, for example, which is necessary for the curing of a photo-curable resin is greatly lowered and it further becomes difficult to form a fully satisfactory image with an economic amount of exposure because the carbon black seizes completely the radical emitted from a photopolymerization initiator by dint of the ultraviolet light and consequently impedes photopolymerization. Further, when the amount of carbon black so added increases, the film ultimately obtained inevitably acquires electroconductivity and becomes no longer usable as a black matrix for a liquid crystal display. Moreover, since carbon black has weak affinity for other substances such as, for example, organic macromolecular compounds, water, and organic solvents as compared with the cohesive force between particles as described above, it incurs extreme difficulty in being uniformly mixed or dispersed in such a photo-curable resinous composition as mentioned above and produces a stable and uniform black layer only with difficulty.
JP-A-04-63870 proposes to use as a coloring agent for incorporation in the photo-curable resinous composition and in combination with carbon black a set of organic pigments of different colors capable of jointly forming a black color so as to repress the otherwise inevitable increase in the amount of carbon black for securing stated imperviousness to light and consequently curb the decrease in the transmittance of light in the ultraviolet zone necessary for the curing with light and enable the film to retain the insulation.
The additional use of these organic pigments, however, inevitably suffers the light transmittance in the visible zone to rise as compared with the sole use of carbon black and makes it difficult to form a black matrix in a small wall thickness with fully satisfactory imperviousness to light and meanwhile offers no solution to the problem of inferior dispersibility of carbon black and the problem of impediment to curing by light in consequence of the seizure of radical by carbon black.
JP-A-06-67421 proposes to adopt, as a black photo-curable resinous composition excellent in photosensitivity and stability of dispersion and consequently appropriate for the fabrication of a black matrix excelling in imperviousness to light and resolution what is obtained by incorporating the carbon black graft polymer as mentioned in (2) above. Since the carbon black graft polymer is not allowed to have a large carbon content as described above, however, it acquires no ample imperviousness to light, allows no satisfactory dispersion of carbon black therein, and betrays deficiency in terms of sensitivity to curing by light and strength of the cured film.
Further, in the formation of a film such as a black matrix with a photo-curable resinous composition of the kind mentioned above, the practice of causing the film obtained after the curing by light and the development of pattern to be subjected to a heat treatment (after-baking) at a temperature in the approximate range of 100-300.degree. C. for the purpose of ensuring thorough curing of the film and, at the same time, exalting the adhesiveness between the substrate and the film is prevalent. The film which has undergone the after-baking treatment is observed to have its resistivity greatly decreased probably because the dispersed state of carbon black or carbon black graft polymer in the film is varied during the course of the heat treatment. Thus, the film is no longer usable as a black matrix for a liquid crystal color filter which demands high resistance.
Also, in the formation of the film as the black matrix with a photo-curable resinous composition, it is favorable to use the resinous composition in an alkali-soluble form. In this case, the capability of the film to be developed with an alkali is degraded by the carbon black or the conventional carbon black graft polymer incorporated as a coloring agent component. The film, therefore, needs further improvement in this respect.
Incidentally, the ink jet printing method accomplishes required recording by forcing a liquid recording medium, i.e. the so-called ink, to fly in a stream of liquid droplets and allowing this stream to impinge on a recording member. The ink jet printing method is known in various types depending on the manner of producing and controlling the droplets of the liquid recording medium. For the production of the liquid droplets, for example, 1 the piezoelectric pressure method which comprises forming part of the wall surface of an ink chamber with a piezo element, giving a recording signal to the piezo element region thereby exerting pressure on the ink in the ink chamber, and causing the ink to emit droplets corresponding to the signal and 2 the thermal pressure method which comprises exerting thermal energy corresponding to a recording signal on the recording liquid (ink) in an ink chamber thereby causing the ink to generate bubbles and produce liquid droplets by dint of the pressure of the bubbles are typical examples of the method already reduced to practical service and, in addition thereto, 3 the method which comprises spouting ink through a nozzle with relatively high pressure thereby allowing the thread of ink in flight to be divided into particles by dint of surface tension and electrification, 4 the method which comprises spouting ink under relatively low pressure and additionally imparting supersonic vibration to the thread of ink in flight, and 5 the method which comprises using meniscuses to form globules of ink and the ink globules with a high electric field have been known.
The ink or liquid recording medium which is used in the ink jet printing method as described above is expected to fulfill various requirements such as having liquid properties (viscosity, surface tension, and electroconductivity) fit for spouting conditions (such as, for example, drive voltage of the piezoelectric element, drive frequency, the shape and material of the spouting orifice, and the diameter of the spouting orifice), tolerating protracted storage stably while inducing no clogging of the ink jet device, attaining quick and infallible fixation to a material for recording (paper or film), producing dots of smooth periphery without notably smearing surrounding areas, forming an ink image of clear and dense color tone, enabling the formed ink image to exhibit outstanding resistance to water and light, having no ability to corrode peripheral materials for ink (storage device, connecting tubes, and sealing material), emitting odor or producing poison only sparingly, and excelling in safety in terms of inflammation.
As the liquid recording medium to be used in the ink jet printing method, the product composed principally of a dye as a coloring agent component and a solvent therefor has been mainly used heretofore. The physical properties of the liquid recording medium, therefore, hinge greatly on the nature which is inherent in the dye to be used. When the ink jet printing is performed with a liquid recording medium containing a water-soluble dye, for example, the ink image which is produced is at a disadvantage in being compelled to sacrifice its resistance to water or light by the physical properties of the water-soluble dye. Besides, the ink itself which contains the water-soluble dye has no ample shelf life.
An attempt is being made to apply a pigment type ink using a pigment like carbon black as a coloring agent component in the place of the dye type ink for the ink jet printing method. This pigment type ink has the advantage that the produced ink image enjoys high satisfactory resistance to light or water as compared with the image formed of the dye type ink. Since the pigment is insoluble in the liquid medium of ink, however, a highly advanced technique is required for finely dispersing the pigment in the liquid medium of ink and unusual difficulty is involved in imparting exalted stability to the dispersion.
The application of the carbon black graft polymer mentioned above as a coloring agent component for the liquid recording medium to be used for the bubble jet printing method is not inconceivable. In this case, the problems mentioned above entrain such other problems as 1 preventing the carbon black graft polymer from enjoying an addition to its carbon black content and the ink image from acquiring a color tone with ample clarity and high density, 2 rendering it difficult for the electroconductivity to be set at a level fitting the discharge condition, and 3 preventing the dispersed state of the carbon black graft polymer from being stably retained and consequently impairing the shelf life of the liquid recording medium.
In recent years, the field of electrophotographic printing, electrostatic copying, etc. has come to devote an increasing effort to the improvement of produced images in quality. The advance of the digital technology has reached a point where the formed latent images warrant high accuracy and fineness as compared with those obtained by the conventional technology. When a powdered toner is used in developing a digital image of electrostatic charge, however, the developed image has no very high quality because the toner particles have particle diameters of the order of 10 .mu.m. In the light of this fact, the formation of an image of enhanced accuracy and fineness due to the use of a liquid developing agent containing toner particles of a smaller particle diameter has come to attract interest recently.
The liquid developing agent for use in the development of an electrostatically charged latent image, as widely known, is produced by dispersing toner particles in an electrically insulating organic liquid such as, for example, an aromatic hydrocarbon type solvent. The toner particles are composed of a pigment, a resin, and other additives because they required to have a coloring property, an electrically charging property, a fixing property, and a dispersion-stabilizing property. In the liquid developing method, the development of an electrostatically charged latent image is attained by charging the toner particles generally to a polarity opposite the polarity of the charge of electrostatic latent image and consequently causing these toner particles to be attracted by the electric charge of the latent image through the electrically insulating liquid, migrated to the latent image electrophoretically, and deposited selectively on the latent image.
The liquid developing agent of this nature is also required to offer an improved ability to disperse a pigment such as carbon black or a resin. JP-A-07-253688, for example, discloses use of a resin component which is obtained by grafting a macromolecular compound soluble in an aromatic hydrocarbon type solvent to a macromolecular compound which is insoluble in the solvent and JP-A-07-104524 has a mention to the effect that such a carbon black graft polymer as mentioned above may be used as a coloring agent. Neither of these inventions, however, are still short of producing a liquid developing agent with a satisfactorily stabilized dispersing property.